posix-timers: Make clock_getres and clock_get mandatory

Richard said: "I would think that we can require k_clocks to provide
the read function. This could be checked and enforced in
register_posix_clock()."

Add checks for clock_getres and clock_get in the register function.

Suggested-by: Richard Cochran <richardcochran@gmail.com>
Cc: John Stultz <johnstul@us.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>